1. Field of the Invention
The present invention relates to an optical information recording medium, particularly to a high-recording-density optical information recording medium such as a Blu-ray disc having an optical recording layer containing an organic dye.
2. Description of the Related Art
Optical information recording media including optical disks have been widely used for information recording. For example, an optical information recording medium such as CD-R has a recording layer and a reflective layer formed in this order on an optically transparent resin substrate with a thickness of 1.2 mm and a diameter of 120 mm or 80 mm. In these years, there has been a demand for an optical information recording medium having a higher information recording density. Thus, use of a shorter laser wavelength (referred to as wavelength shortening) and use of an object lens having a higher numerical aperture (NA) (referred to as NA increasing) have been studied, and DVD±R and the like have been realized as such media. The DVD±R has two bonded 0.6-mm-thick optically transparent resin substrates, and has a reflective layer and a recording layer sandwiched between the substrates, so that the permissive tilt angle (tilt) value of the disc is increased by the wavelength shortening and the NA increasing.
Recently there has been a demand for a further higher information recording density in view of recording a high-definition viewdata. Thus, an optical information recording medium having a 1.1-mm-thick resin substrate, a reflective layer and a recording layer formed on the substrate, and a 0.1-mm-thick optically transparent cover layer formed on the reflective and recording layers has been proposed. Examples of the medium include a write-once type of Blu-ray Disc (registered trademark, hereinafter referred to as BD), BD-R (BD-Rewritable). Specifically, the medium has a disc-shaped substrate having an outer diameter of 120 mm, an inner diameter of 15 mm, and a thickness of approximately 1.1 mm, and further has an optical reflection layer, a phase change recording layer, and a cover layer having a thickness of approximately 0.1 mm on the substrate. The medium is capable of recording user information on the recording layer by irradiating the cover layer side with a laser light having a wavelength of approximately 405 nm from an optical head having a numerical aperture of approximately 0.85.
Though the recording layer of the above optical information recording medium is a phase change type layer, also a dye-containing type recording layer has been studied.
However, the dye-containing recording layer is disadvantageous in that, for example, when a transparent resin material is applied to the recording layer to form the cover layer, the coating liquid penetrates into the dye-containing recording layer.
In contrast, as shown in FIG. 5, in an optical recording medium proposed in JP-A-2003-36562, a reflective layer 2, a dye-containing recording layer 3, and a cover layer 5 are disposed on a substrate 1 to achieve recording/playback on the cover layer 5 side, and a barrier layer 4 for preventing mixing of the recording layer 3 and the cover layer 5 is formed between the recording layer 3 and the cover layer 5. The barrier layer 4 may contain at least one selected from silicon oxide, zinc sulfide, zinc oxide, silicon nitride, silicon carbide, cerium oxide, yttrium oxide, yttrium sulfide, and mixtures of an oxide and sulfur.
However, as described in JP-A-2003-36562, the optical recording medium is disadvantageous in that when the barrier layer 4 contains a sulfur-containing material, the sulfur is reacted with a metal (such as Ag) in the reflective layer 2, so that a sulfide is generated to deteriorate the preserving property.
Meanwhile, in an optical recording medium proposed in JP-A-2006-147135, at least a cover layer and a recording layer containing an organic dye are disposed on a substrate, and an interlayer with a thickness of 1 to 80 nm, composed of a Ta compound, an Nb compound, or a composite oxide containing Ta and/or Nb, is disposed between the recording layer and the cover layer. The interlayer may be formed by an RF sputtering method or a DC sputtering method.
However, the optical information recording medium proposed in JP-A-2006-147135, which has the interlayer composed of the Ta compound, the Nb compound, or the composite oxide containing Ta and/or Nb such as a composite oxide containing 70 atm % of Nb2O5 and 30 atm % of Al2O3, has the following disadvantages. Specifically, the electric conductivity of a target of the composite oxide containing 70 atm % of Nb2O5 and 30 atm % of Al2O3 is changed depending on the oxygen deficiency state of the target. For example, in a case where the target has no oxygen deficiency, the target is a perfect insulator, whereby the interlayer can be formed by the RF sputtering method using the target only at a low film formation speed on the dye-containing recording layer. Thus, in this case, the optical information recording medium cannot be easily produced, and cannot be practically used in view of the production. On the other hand, in a case where the target has oxygen deficiency and thereby has electric conductivity, abnormal discharge is often caused between the target and the interlayer in an RF sputtering machine under a high-frequency voltage applied between a vacuum chamber and the target. In this case, a groove-like scratch is generated on a surface of the interlayer composed of the composite oxide due to the abnormal discharge, and is found as an appearance failure in the optical information recording medium after forming the cover layer. Furthermore, immediately before and after the abnormal discharge, the voltage applied to the target is changed, whereby the interlayer cannot be formed at a constant speed, the resultant interlayer has an uneven film state, and the optical information recording medium cannot be stably produced.